Production of hydrogen by utilization of thermochemical cycles has been a widely studied field since the late 60 s and has been subject to systematic evaluation of the elements which may be involved.
Among these thermochemical cycles, hydrogen production cycles based on oxidation-reduction of cerium, and in particular on the cerium-chlorine system have attracted attention.
The thermochemical cycle based on the oxidation-reduction of cerium may be described by the following reactions, according to a first alternative:H2O+Cl2=2HCl+½O2;  (A)8HCl+2CeO2=2CeCl3+Cl2+4H2O;  (B)2CeCl3+4H2O=2CeO2+6HCl+H2.  (C)
According to a second alternative, this thermochemical cycle may be described by the following reactions.H2O+Cl2=2HCl+½O2;  (A)8HCl+2CeO2=2CeCl3+Cl2+4H2O;  (B)2CeCl3+2H2O=2CeOCl+4HCl;  (B′)2CeOCl+2H2O=2CeO2+2HCl+H2.  (C′)
The document of C. E. BAMBERGER <<Hydrogen Production from Water by Thermochemical Cycles>>, Cryogenics, March 1978, p. 170-182, [1] gives a list of 129 thermochemical cycles having been the subject of publications between October 1975 and September 1977. This document supplements a first list of 72 thermochemical cycles published in 1976 in the document of BAMBERGER C. E., RICHARDSON D. M., Cryogenics, 16 (1976), p. 197, [2].
Among these 129 cycles, cycles are mentioned which involve cerium chloride according to the two mentioned alternatives (cycles No. 30 and No. 31, p. 173, of the first document of BAMBERGER).
The document of C. M. HOLLABAUGH, E. I. ONSTOTT, T. C. WALLACE Sr., and M. G. BOWMAN, <<A study of the cerium-chlorine system for thermochemical production of hydrogen>> [3] studies in detail the cycle involving cerium chloride.
The document of C. M. HOLLABAUGH is focused on the second alternative of the cerium chloride cycle and in particular on the three last reactions (B), (B′), (C′) of the cycle, mentioned above.
These reactions are all heterogeneous reactions in which a gas (HCl or H2O) comes into contact with a solid (CeO2, CeCl3 or CeOCl), reacts with it and transforms it into a solid product (CeCl3, CeOCl, or CeO2). The first reaction, (A), which is not studied in the HOLLABAUGH document, as for it, only involves gases.
The fact that the reactions involved are in this document, all conducted in a solid-gas phase or in a gas-gas phase leads inter alia to the following difficulties and problems:                a limitation of the degrees of progression of the reactions by passivation of the surfaces, thus the work carried out by HOLLABAUGH et al. [3] shows that at best a progression coefficient of 0.3 is attained after 50 minutes;        a limitation of the reaction kinetics by diffusion of gas species inside the solids;        heating of solid particles which is sometimes difficult to achieve;        several transports of solids to be carried out for conveying the species from one reactor to the other;        a significant modification of the molar volumes of the solid CeO2 and CeCl3 with a 260% increase between the two, which has the consequence that the stability of the conversion reactors is difficult to maintain.        
The same problems are posed as regards the first alternative of the thermochemical cycle of cerium which involves the same reactions (A) and (B).
The conclusion had been drawn that essential problems are posed during the application of the oxidation-reduction cycle of cerium both in its first and its second alternative, in terms of reactivity and control of the process.
A method for producing hydrogen via a thermochemical route from water, based on oxidation-reduction of cerium and more specifically on the cerium-chlorine cycle, with which it is possible to solve the problems of the methods described in documents [1], [2] and [3] mentioned above, is proposed in document FR-A-2 880 012 [4].
In the method of document [4], the reaction (b) is conducted in a liquid phase, more specifically in a solution of hydrochloric acid, and in which CeO2 is introduced in solid form.
However, it was found that the hydrochlorination reactions (B) of each of the reaction schemes depended on a wide proportion of the cerium oxide batches used and that, if certain batches gave excellent results, other batches on the contrary gave poor results or even bad results in terms of dissolution of CeO2.
In other words, it was observed that for certain batches, dissolution of CeO2, although existent, was immensely slow and that it therefore considerably limited the efficiency and the overall yield of the method.
Regarding the foregoing, there consequently exists a need for a method for producing hydrogen via a thermochemical route from water, based on the chlorine-cerium cycle, in which the hydrochlorination reaction (B) of cerium oxide is conducted, like in document [4], in a liquid phase, which allows global improvement in the kinetics of the reaction (B) or dissolution reaction, and which ensures that this dissolution reaction is effective and reliable.
There still exists a need for such a method, in which the dissolution reaction (B) may actually be applied with high reliability and great reproducibility regardless of the origin of the CeO2 and regardless of the cerium oxide batch used.
In other words, there exists a need for a method for producing hydrogen via a thermochemical route from water, based on the chlorine-cerium cycle, in which the reaction (B) for hydrochlorination of cerium oxide is conducted, like in document [4], in a liquid phase, which has all the advantages of the method subject of document [4], widely described in the latter, but which does not have the drawbacks thereof, essentially related to the randomness of the dissolution kinetics of CeO2 during step (B), and to the very slow dissolution rate of cerium oxide often observed during the reaction (B).
The goal of the present invention is to provide a method for producing hydrogen via a thermochemical route from water, based on the chlorine-cerium cycle, in which the reaction (B) for hydrochlorination of cerium oxide is conducted like in document [4] in a liquid phase, which inter alia meets the needs listed above.